1. Field of the Invention
The present invention relates to a holographic data recording medium, and more particularly, to a system and a method for recording and reading holographic data.
2. Description of the Related Art
Variously-proposed holographic data recording systems commonly include recording many pages of data at one or more locations within a holographic recording medium. A plurality of pages of data can be multiplied and be recorded at a single location by changing a characteristic of the reference beam used for recording, such as illuminating angle or wavelength of the reference beam for recording. A spatial light modulator (SLM) such as a liquid crystal display panel is typically used to imprint data to be recorded onto a signal beam. To read out the hologram recorded to the holographic recording medium, a reference beam for reading with the same characteristics as the one for recording is used to retrieve the specific hologram, and the data page shown as the reconstructed image of the hologram is read with a two-dimensional detector array, such as a CCD (charge-coupled device) or CMOS (complementary metal oxide semiconductor) array.
The schematically-described holographic data recording systems suffer from the following limitations. That is, the recording density of data in the page is limited by the pixel size and pitch that can be provided by the SLM and detector array and, to compensate for this, hundreds or thousands of holograms need to be multiplexed at each recording location in the holographic recording medium in order to achieve high recording density thereof. Depending on the method of multiplexing used and the amount of overlap between spatial locations in the recording medium, the shape of the volume occupied by each hologram will vary, as will the effective number of holograms multiplexed at each point in the recording medium. Both of these factors can result in nonuniform reading intensities among different holograms in the single recording medium, as well as among different pixels in a hologram, requiring complex compensation techniques to reduce error probabilities in reading the data. If spatial locations do not overlap in the recording medium, then dynamic range is lost in the unused material between discrete locations.
Furthermore, there is currently no feasible method for the fast replication of holographic memory recorded with such holographic data recording systems. For the above-mentioned replication of holographic memory, each hologram must be recorded sequentially for each multiplexed page and at every spatial location in the recording medium. Complicated exposure steps must be repeated at every recording location in order to obtain holograms of equal intensities.
Holographic recording and readout systems that avoid many of these problems and can be quickly replicated by using full-disc holograms for recording images on the disc without any change as the holograms were proposed by the present inventor. A more specific disc recording system was proposed by the present inventor, using a conical optical element to produce a reference beam for recording that illuminates the disc at a constant radial angle at all positions on the disc holographic recording medium. An alternate disc holographic recording system was then proposed by the present inventor, using a spherical reference beam for recording instead of the conical reference beam for recording (refer to Japanese Unexamined Patent Application Publication No. 2001-23169, Japanese Unexamined Patent Application Publication No. 2002-207412, and U.S. Patent Publication No. 2003-0161246).
Examples of typical conventional-art full-disc hologram recording and readout systems are illustrated in FIGS. 7 and 8, respectively. FIG. 7 illustrates a process for recording data mask patterns to a single-volume holographic disc recording medium 1. Referring to FIG. 7, copies of multiple data mask patterns are recorded to the holographic disc recording medium 1 by exchanging the data mask and repeating the recording process. Illuminating each mask with a normally incident planewave, in the recording process, the diffracted pattern is recorded in an adjacently placed transmission-type data mask 2 of the holographic disc recording medium 1, by vertically illuminating a plane beam 5 from the top in FIG. 7 and by illuminating a conical reference beam 3 for recording incident from the opposite side (bottom in FIG. 7) shaped by a conical mirror 4 of the holographic disc recording medium 1 to record holograms in the reflection geometry. A conical beam is shaped to approximate a planewave reference beam with a constant radial angle at all positions on the holographic disc recording medium 1 and input the reference beam. Upon reading the holograms, referring to FIG. 8, the hologram at the illuminating points of the entire ones can be partly read by locally illuminating a reference beam 6 for reading with small beam diameter serving a planewave at a fixed incident angle in the radial direction while the hologram disc recording medium 1 is rotated. The illumination of reference beam 6 for reading propagating in the inverse direction of the reference beam 3 for recording produces a real image of the pattern of the recorded data mask 2 around the point of readout (illuminating point). Likely the reading system of the conventional optical discs, a real image of the reconstructed image is formed on a detector 8 by using an optical system for forming an image including an objective lens 7. The data may be read once from a single track forming the pattern of the data mask 2, or may be read once from the two or more tracks in parallel therewith in a multi-track format for parallely reading the format of the data mask 2. Multiple data layers may be multiplexed by following two multiplexing methods. That is, according to an incident-angle multiplexing method, serving as one multiplexing method, the conical mirror 4 is exchanged every multiplexing and recording a plurality of data masks to change the incident angle of the reference beam for recording. According to a wavelength multiplexing method serving as the other multiplexing method, the wavelength of the reference beam for recording is changed by using a wavelength changing laser.
These proposed full-disc hologram systems have generally assumed that the data patterns recorded to the holographic recording medium are in the form of spiral tracks of pixels, thus serially reading-out the reference beam for reading under continuous illumination, similar to the manner for reading pits from a conventional optical disc such as CD (compact disc) or DVD (digital versatile disc). However, this reading system does not take advantage of one of the inherent benefits of holography, which is potential for increased transfer rate through parallel readout. Also, the continuous serial readout of pixels prevents the use of CCD or CMOS detectors, which integrate detected photons over a short period of time in order to achieve higher signal-to-noise ratio (SNR) as compared with that of photodiodes used for serial detection systems like for conventional optical discs. Holograms also tend to have intensity variations within the bright and dark areas of the hologram image, which increases jitter in the location of bright-to-dark- and dark-to-bright transitions in a serially read signal from a reconstructed image of the hologram. On the contrary, small reading-intensity variations are less of a problem with CCD and CMOS detectors, for which noise can be reduced by averaging over pixel areas and thresholds can be defined to distinguish bright and dark pixels. Although the use of parallel readout with CCD or CMOS detector is common in the field of holographic data recording technology, it has only been used for reading small holograms recorded locally over a disc surface (not with full-disc hologram systems), and such systems are not suitable for fast replication.
Also, the holographic disc reading system of previous full-disc hologram one designs have primarily used planewave beams with a small beam diameter, which preferably match the recording conical beam wavefront shape when only a small spot area thereof for illuminating the surface of the holographic recording medium is considered. However, the mismatch becomes more problematic when a large readout spot size is needed between the wavefront shape of the reference beam for reading containing the plane beam and that of the reference beam for recording containing the conical beam, such as when a spatial frequency of the recorded data is very high, there is a relatively long diffraction distance between the data pattern and holographic recording medium during recording the holograms, the holographic recording medium is thick, or when it is desirable to read out a large portion of the recorded hologram and read the data from the read image.